In the gas-diffusion electrodes of fuel cells, a fuel gas such as hydrogen and an oxidant gas such as air react electrochemically, so as to supply electricity and heat simultaneously. Owing to the variety of electrolytes with which they are equipped, there are several types of fuel cells.
Polymer-electrolyte fuel cells are furnished with electrolyte membrane-electrode assemblies (MEAs), made up of a polymer-electrolyte membrane and a pair of gas-diffusion electrodes sandwiching the membrane. The polymer-electrolyte membrane for example has a —CF2— skeleton and comprises a perfluorocarbon sulfonic acid having sulfonic acids on the terminal ends of its side chains.
The gas-diffusion electrodes comprise a catalyst layer contiguous with the polymer-electrolyte membrane and, arranged on the outer face of the catalyst layer, an electrode substrate having gas-permeable and electroconductive properties. The catalyst layer comprises a carbon powder carrying a platinum-system metal.
An electroconductive separator for affixing an MEA, and at the same time electrically interconnecting in series neighboring MEAs, is arranged on the outer face of the MEA. The electroconductive separator has a gas-supplying channel for the fuel gas or the oxidant gas to the gas-diffusion electrodes, and for conveying a surplus gas and water created by the reaction of hydrogen and oxygen.
Seals such as gaskets are arranged encompassing the gas channels on the electroconductive separator and the gas-diffusion electrodes, and they prevent intermixing and outward leakage of the gases.
To heighten output voltage in procuring power-generating devices, a plurality of individual single cells, composed of an MEA and a pair of electroconductive separators having gas channels, are laminated. A fuel gas and an oxidant gas are supplied from the exterior through a manifold to each gas channel. Electric current generated through the electrode reactions is then collected at the electrode substrates and taken out to the exterior through the electroconductive separator.
Electroconductive separators are often made from a carbon material having gas-tight and anticorrosive properties. Likewise, electroconductive separators utilizing a metal substrate such as stainless steel have been investigated from the viewpoints of manufacturing cost, as well as ease of working and thinning the electroconductive separator.
Due to long-term exposure to high-humidity gases, electroconductive separators utilizing metal substrates require strong corrosion resistance. Furthermore, in order to heighten the electric cells' power-generating efficiency, suppressing contact resistance between the electroconductive separator and the MEA also becomes important. Therein, if for example a stainless-steel sheet is utilized as a metal substrate, by forming a passive state layer consisting of chromium oxide on the obverse face of the stainless-steel sheet, its corrosion resistance is heightened.
Nevertheless, because forming the comparatively thick and stable passive state layer on the obverse face of the metal substrate makes the passive state layer an electrical resistor, the contact resistance increases. High-output cells consequently cannot be procured. Conversely, if the passive state layer is unstable, the metal substrate will corrode, and the MEA will undergo damage due to the metal ions leached.
A method of establishing on the obverse face of the metal substrate a layer obtained by means of chemical-plating or vapor-depositing an anticorrosive metal such as gold has been investigated. Lowering costs, however, is difficult.
A method of coating onto the obverse face of the metal substrate a resin composition in which a carbon powder is dispersed into cellulose, poly(vinyl chloride), epoxy resin, etc. has also been investigated. Nevertheless, there are problems with the durability.
A method of forming a layer on the obverse face of the metal substrate in order to heighten its corrosion resistance, and meanwhile arranging, in the area where the separator and the MEA contact, an electroconductive particulate substance of high hardness in order to form an electroconductive path by penetrating the aforementioned layer, has also been investigated. This method is comparatively low-cost. Nevertheless, when the cells are run long-term, in the end metal ions leach out and the MEA undergoes damage.